The two goals of this unit are to further define the ramifications of dystrophin-deficiency in skeletal muscle and to assess the ability of exogenous genes coding for full length or truncated dystrophin proteins to prevent the injury that is concomitant with loss of dystrophin. These studies will utilize the mdx mouse as the animal model for dystrophin- deficiency. By delivering dystrophin cDNAs either with adenovirus or via germline integration, under the control of different promoters, we will also assess the relationship between the level of protein expression and the degree of protection from contraction-induced injury. In addition, we will investigate the general mechanism of contraction-induced injury, a process which we postulate is exacerbated by the lack of dystrophin. The long-term goal is to gain the understanding and tools necessary to demonstrate the feasibility of adenoviral-based gene therapy for Duchenne muscular dystrophy. Despite the potential for successful transfection of mdx muscle fibers with adenovirus containing the dystrophin mini-gene and their functional restoration following expression of dystrophin, it is at present unclear whether this will lead to compensatory restoration of motor innervation. Preliminary observations suggest that neuromuscular innervation is highly unstable in muscles of mdx mice. While the cause of this instability is likely to be repeated cycles of muscle fiber degeneration and regeneration which occur over time in mdx mice, we wish to determine whether motor innervation can be restored to a level which is functionally appropriate for recovery of motor output. Thus we will examine synaptic maintenance on normal, de- and regenerating muscle fibers in untransfected and transfected mdx mice by following neuromuscular synapses over time in living mice. Functional analyses will include characterization of neuromuscular synaptic transmission and of motor units.